Ubuntu releases are year.month in nature, standard releases have a 9 month supported life, so 19.04 from 2019-April is only a week away from becoming EOL thus it's not your best choice. 19.04 will release-upgrade to 19.10 but for some people these 6-9 month upgrade cycles are too much hassle, so LTS or long-term-support releases are better choices.
Ubuntu LTS releases have support for 5 years, flavor though only 3 years. LTS releases are the first release of the even year, so Ubuntu 18.04 LTS is the most recent; ie. 2018-April release of Ubuntu.
Flavors of Ubuntu are the same Ubuntu based; but a different desktop. My q9400 (q2d) cpu will run Ubuntu fine, but I get a far better experience using XFCE (Xubuntu) or LXQt (Lubuntu), so my choice is Lubuntu (LXQt).
Software choices also play a part, as does what your user preferences are. So you must decide for yourself what best suits you. Ubuntu has many choices.
(Software choices; modern Lubuntu uses the Qt5 toolkit, so Qt based apps will be more efficient, where as modern Xubuntu uses GTK+3 so GNOME apps will be more efficient using Xubuntu. If you have enough ram & can afford to have multiple toolkits in memory that do the same thing you can ignore this - as windows users do anyway (Qt is available & used in windows software too; users of windows/mac just get told they next more ram without the why). With 3gb of RAM ; I'd consider the toolkit/libraries/software choices in your decision; my general rule of thumb is you can ignore the RAM issue if you have 4gb+, but it's far more critical on machines with 2gb only.
Best of all - you can try Ubuntu (and flavors) before you install (this document assumes Ubuntu, but works equally well with other flavors, just GUI/desktop will differ)
https://tutorials.ubuntu.com/tutorial/try-ubuntu-before-you-install#0
